1. Field of the Invention
The present invention relates to an active-matrix liquid crystal display (AM LCD) device, and more particularly, to a field sequential liquid crystal display device and a method of color image display for the field sequential liquid crystal display device. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for improving a field sequential liquid crystal display device leading to an increase of instantaneous luminance of specific color and a decrease of response time of a liquid crystal, for example.
2. Discussion of the Related Art
Until now, the cathode-ray tube (CRT) has been generally used for display systems. However, flat panel displays are increasingly beginning to be used because of their small depth dimensions, desirably low weight, and low power consumption. Presently, thin film transistor-liquid crystal displays (TFT-LCDs) have been developed with a high resolution and small depth dimensions.
Generally, a liquid crystal display (LCD) device includes an upper substrate, a lower substrate, and a liquid crystal layer interposed between the upper and lower substrates. The upper and lower substrates respectively have electrodes opposing to each other. When an electric field is applied between the electrodes of the upper substrate and the electrodes of the lower substrate, molecules of the liquid crystal are aligned according to the electric field. By controlling the electric field, the liquid crystal display device provides varying transmittance of the light of incident to the display images.
Currently, an active-matrix liquid crystal display (AM LCD) device is the most popular because of its high resolution and superiority in displaying moving images. A typical active-matrix liquid crystal display has a plurality of switching elements and pixel electrodes, which are arranged in an array matrix on the lower substrate. Therefore, the lower substrate of the active-matrix liquid crystal display is alternatively referred to as an array substrate.
The structure of a conventional active-matrix liquid crystal display will be described hereinafter with reference to FIG. 1, which illustrates a cross-section of a pixel region. The active-matrix liquid crystal display 10 consists of a liquid crystal panel 15 and back light 50. The liquid crystal panel 15 includes a color filter substrate (an upper substrate) 20 and an array substrate (a lower substrate) 40 which face each other across a liquid crystal layer 30. A color filter layer 22, which includes a black matrix 22b for excluding a leakage of light and sub-color-filters 22a, consisting of red (R), green (G), and blue (B), is formed on the color filter substrate 20. A common electrode 24 is formed on the color filter layer 22 as one of electrodes for applying a voltage to the liquid crystal layer 30. A thin film transistor, for functioning as a switching element, and a pixel region are formed on the array substrate 40 facing the color filter substrate 20. A pixel electrode 42, electrically connected to the thin film transistor and functioning as another electrode in applying a voltage to the liquid crystal layer 30, is formed on the array substrate 40. The back light 50 is disposed under the array substrate 40 to irradiate light to the liquid crystal panel 15. This liquid crystal display device uses optical anisotropy and polarization properties of liquid crystal molecules for displaying a desired image. That is, applying a voltage to the liquid crystal molecules having a thin and long structure and a pretilt angle changes an alignment direction of the liquid crystal molecules. Thereafter, incident light from the back light is polarized due to the optical anisotropy of the liquid crystal molecules. And lastly, the polarized light is modulated by passing through the color filter layer and thus color images are displayed. The thin film transistor includes a gate electrode and a source and a drain electrodes (not shown).
But the conventional active-matrix liquid crystal display device has some problems. First, the material used for the color filter is expensive and the methods for manufacturing the color filter require more material to be consumed in the manufacturing process, resulting in an increase in the manufacturing cost. Second, the maximum value of a transmissivity of a material used for the color filter is 33%, so that a brighter back light needs to be used in order to display a color image effectively, which results in the increase of the power consumption. Last, when the color filter is thick, properties of color are fine, but the transmissivity is decreased. On the other hand, when the color filter is thin the transmissivity can be raised but, the color properties will become poor. Therefore, a manufacturing process having great precision is required for the color filter, which results in a decrease in production yield and an increase in the rate of inferior goods.
Many studies and experiments have been conducted recently, and a field sequential liquid crystal display device, able to display a full color without the color filter, is suggested as an alternative. The field sequential liquid crystal display devices display a color image by turning on light sources Red, Green and Blue sequentially during a frame, whereas the conventional active-matrix liquid crystal display devices display the color image by a white light source of the back light that is constantly turned on. The field sequential liquid crystal display device has not been popular until recently because of poor response time. However, development of new liquid crystal modes such as Ferroelectric Liquid Crystal (FLC), Optical Compensated Birefringent (OCB) and Twisted Nematic (TN) having a high response time of the liquid crystal can result in more wide spread use of the field sequential liquid crystal. In addition, the Optical Compensated Birefringent (OCB) mode is generally used for the field sequential liquid crystal display device. Both surfaces of an upper and a lower substrates are rubbed in a same direction and thereafter a voltage is applied to form a band-structure of the liquid crystal in OCB mode. Because the movement of liquid crystal molecules becomes faster when the voltage is applied to the liquid crystal, the response time of the liquid crystal becomes fast-within about 5 m/sec. Accordingly, the liquid crystal cell of the OCB mode is suitable for the field sequential liquid crystal display device because of its high response time leaving no residual image on a screen.
FIG. 2 is a cross-sectional view illustrating the schematic cross section of the conventional field sequential liquid crystal display device. The conventional field sequential liquid crystal display device 60 includes an upper substrate 64 (referred to as a color filter substrate), a lower substrate 66 (referred to as an array substrate), a liquid crystal layer 70 interposed the upper and lower substrates and a back light 72 consisting of three light sources Red, Green and Blue to irradiate light to the liquid crystal panel 62. A black matrix 61 is formed between the common electrode 65 and a transparent substrate 1 of the upper substrate 64 in order to intercept light in a region other than a region of the common electrode 67. A thin film transistor functioning as a switching element and electrically connected to the pixel electrode is formed on the lower substrate 66. The thin film transistor consists of a gate electrode and a source electrode and a drain electrode (not shown). The major difference of the field sequential liquid crystal display device 60 with the previous conventional liquid crystal display is that the field sequential liquid crystal display device does not need the color filters and has the back light having three light sources selectively turned on and off. The light sources Red, Green and Blue are driven respectively by an inverter (not shown) and each of light sources Red, Green and Blue is turned on and off one hundred and eighty times per second, and thus a color image is displayed using a residual image effect of eyes caused by the mixture of three colors, red, green and blue. Even though the light source is turned on and off one hundred and eighty time per second, to the naked eye the light source appears to be kept on. For example, if the light source Red is turned on and then the light source Blue is turned on, a mixed color violet is seen owing to the residual image effect. Whereas a total luminance of the conventional active-matrix liquid crystal display device is low owing to the low transmissivity of the color filter, the field sequential liquid crystal display device overcomes this problem because it does not have a color filter. In addition, the field sequential liquid crystal display device is suitable for a large scale liquid crystal display device because it can display a full-color using three color light sources, whereby it can display an image of high luminance and high resolution. Even though the conventional active-matrix liquid crystal display device is inferior to CRT (Cathode Ray Tube) in terms of price or clearness, the field sequential liquid crystal display device can settle this problems.
FIG. 3A is a cross-sectional view illustrating a wave guide type back light of the field sequential liquid crystal display device; FIG. 3B is a cross-sectional views illustrating a directly underlaid type back light of the field sequential liquid crystal display device. The wave guide type back light has light sources Red, Green and Blue disposed in a row at one edge or both edges of the liquid crystal panel 62 and diffuses light using a light guide panel and reflector. The wave guide type back light 74 may use a Cold Cathode Fluorescent Lamp (CCFL) as a light source and is suitable for notebook computers or the like because of its low weight and power consumption. The directly underlaid type back light 76 has light sources Red, Green and Blue 75 disposed in a repeated sequence of Red, Green and Blue under a scattering film 77 and irradiates light directly to the whole surface of the liquid crystal panel 62. The directly underlaid type back light is usually used for the image display device where the luminance is important and has a high power consumption because of its relatively big thickness and high ratio of diffusion.
FIG. 4A is a plane view showing a part of an array substrate. A plurality of horizontal gate bus lines 78 and vertical data bus lines 80 crossing gate bus lines are formed on the array substrate, and a thin film transistor is formed at every intersection of gate bus lines and data bus lines. A pixel electrode 79 electrically connected to the thin film transistor is formed on the array substrate. The conventional field sequential liquid crystal display device is driven by applying an image signal data to the data line 80 and scanning an electric pulse to the gate line 78. A line sequential driving method is used for the field sequential liquid crystal display device in order to improve a quality of an image, where a gate scan input driver applies a gate pulse voltage to one of gate lines at a time and applies the gate pulse voltage sequentially to the next gate line. One frame is completed when the gate pulse voltage is applied to all gate lines. That is, if the gate pulse voltage is applied to nth gate line 78, all of thin film transistors connected to the nth gate line 78 are turned on, and the image signal of the data line 80 is accumulated in liquid cells and in storage capacitors through the thin film transistor that have been turned on. Accordingly, liquid crystal molecules are realigned according to the image signal data accumulated in the liquid crystal cell and an image signal voltage, and then a desired image is displayed after the light from the back light passes through the liquid crystal cell.
FIG. 4B is a time chart showing a driving method of the conventional field sequential liquid crystal display device. The driving sequence of the conventional field sequential liquid crystal display device is as follows. After all thin film transistors for one of the light sources are turned on sequentially, the liquid crystal molecules become aligned according to the applied voltage, and then the next one of light sources is turned on. And the same process is repeated for other remaining light sources. Each of light sources Red, Green and Blue is driven one time respectively for a frame. The driving process of each of the light sources must be completed respectively within one period of sub-frame, i.e. ¼f. Taking one of light sources for example, a period of a sub-frame consists of a scanning time, a response time of the liquid crystal and a flashing time of the back light, and this relation can numerically be expressed as follow:¼f=tTFT+tLC+tBL where f is a frame frequency, tTFT (92) is a scanning time for all thin film transistors of sub-frame, tLC (94) is a response time of the assigned liquid crystal and tBL (96) is a flash time of the back light. If the frame frequency tTFT (92) is increased, whereas the flash time tBL (96) is kept constant, the response time tLC (94) decreases because the time period of one sub-frame is fixed. If the response time tLC (94) is decreased, and thus an actual response time of the liquid crystal becomes longer than the assigned response time of the liquid crystal, the back light is driven before the proper alignment of the liquid crystal occur, causing screen color to not be uniform.
FIG. 5 is a schematic diagram illustrating a sequence of color image display for one frame in the conventional field sequential liquid crystal display device. The one frame period of the field sequential liquid crystal display device is 1/60 second, and the sub-frame period for each of the light sources Red, Green and Blue is one-third of the one frame period, i.e., 1/180 second (5.5 msec). The actual lighting time of each of light sources Red, Green and Blue for a sub-frame becomes shorter than 1/180 second because color interference may happen when light sources Red, Green and Blue are driven as on-state continuously. As shown in the figure, a sequence for color image display for the field sequential liquid crystal display device is as follow. One frame “F” is divided into three sub-frames S1, S2 and S3 for each of the light sources Red, Green and Blue, and each of the light sources is sequentially turned on and off in order to display a color image by irradiating light to the liquid crystal panel (62).
FIG. 6 is a schematic diagram illustrating a sequence of color image display for one frame in the conventional field sequential digital light processing (DLP) device used for a projector, for example. The field sequential digital light processing device uses four light sources Red, Green, Blue and White. Because the field sequential digital light processing device irradiates light using a principle of reflection of mirror, it has a high efficiency of use of light and can display an image of higher luminance than a transmissive type of liquid crystal display device irradiating light from behind the liquid crystal panel. Because every control is accomplished digitally, and the device has a single plate structure, it is suitable for minimization of products. The field sequential digital light processing device controls a refraction ratio using an non-light emitting element instead of the liquid crystal. As shown in the figure, one frame “F” is divided into four sub-frames Sa, Sb, Sc and Sd for each of light sources Red, Green, Blue and White. Each of light sources is sequentially turned on and off in order to display a color image by irradiating light to the digital light processing panel (82). One frame period of the field sequential digital light processing device is 1/60 second, and the sub-frame period for each of the light sources Red, Green, Blue and White is one-fourth of the one frame period, i.e., 1/240 second.